1. Field of the Invention
The present invention relates to a process and an apparatus for smelting zinc and, more particularly, to a process for advantageously recovering zinc from zinc-containing materials by electrothermic distillation, and an apparatus for carrying out the said process.
2. Description of the Prior Art
It is known that zinc content in zinc-containing materials containing oxidic zinc such as zinc oxide, zinc ferrite, zinc silicate and zinc carbonate, for example, roasted zinc ore, leach residue from the hydrometallurgical refining of zinc, zinc-containing flue dust of steel and iron production and so on, can be recovered by converting them into sintered ore and thereafter subjecting them to electrothermic distillation. According to continuous electrothermic distillation under the use of a shaft-type electrothermically distilling furnace, the zinc-containing material and others are introduced continuously into the top of the furnace and then an electric current is conducted through this furnace charge to proceed with reductive distillation by the Joule's heat, while discharging the so treated furnace residue from lowest part of the furnace. The continuous distillation method described above has such advantages as higher yield of zinc recovery, higher efficiency of energy utilization and so on.
In such electrothermic distillation method, zinc-containing starting material is usually used, which is obtained by sintering and briquetting raw zinc-containing materials available generally in pulverous form and then subjecting them to dressing or size regulating so as to attain strength and gas permeability suitable to be used as the furnace charge as well as uniform subsidence in the furnace. An equal amount of coke lumps with suitable grain size is simultaneously introduced as a reducing agent for preventing sinters or briquettes from adhering to each other and in order to obtain increased electric conductivity and to maintain a suitable electric resistance. This method of using the sinters or briquettes has however a shortcoming in that the reaction rate of reductive distillation is controlled by the diffusion of the reaction-participating substances, for example, carbon monoxide, carbon dioxide, zinc and so on, within the sinters or briquettes, and therefore, it takes a relatively longer period of time for this reaction, i.e., in other words, only a relatively low productivity can thereby be achieved. The method also shows difficulties such as a larger amount of recycling ore or coke, a larger consumption of coke, necessity of the crushing and size regulating steps in the preparation of the sinters or briquettes and so on. In addition, it becomes also a problem that it may not be avoided to take countermeasures against accompanying dust evolution and environmental deterioration due to the possible sulfur content in the flue gas.
Attempts have been made to eliminate the sintering process and to use the briquettes in which the zinc-containing material is merely mixed with pulverized coke. Also for the binding agent to impart to the briquettes strength capable of withstanding the operating conditions, many have been investigated, however, few could have been found their use for the charge of the shaft-type electrothermically distilling furnace, because of lack of electric conductivity.
On the other hand, a method has also been known, in which briquetted ore obtained from mixture of roasted zinc ore, coal fine and cement mortar etc. is subjected to dry distillation before it is treated by reductive distillation in a vertical retort under external heating by utilizing its higher thermal conductivity. This method, however, owing to the restriction from the heat transfer distance and the thus required limitation of the scale of practical apparatus, cannot be realized in a practical furnace having large capacity. Moreover, a further disadvantage comes forth by the inferior thermal efficiency due to external heating, so that an unprofitable results may be anticipated when applied for, in particular, low-grade zinc-containing material.